Random volume fluctuations on JCM900

I've been noticing these random volume fluctuations when playing through my JCM900 100W 2X12 combo. It seems to only manifest when on the overdrive mode, yet sometime it would work perfectly without a glitch (which adds to the confusion)!

Now, I understand it's an old amp (15+ years). But it has been well looked after and all. I don't suspect the tubes as they have failed in the past and it sounds nothing like this (usually just sounds muffed if tubes fail). It is definitely not the guitar as I have other amps without this issue.

Not just a simple clean required is it? If so, I don't want to take it to the shop and end up paying $100 for something I could do in 5 minutes. Any advice on which bits to focus the cleaning on?

Any help/advice would be very much appreciated.

Oh, and it is summer here and our studio doesn't have great air-con so might overheating be a possibility?

Thats happened to me on my orange rockverb it was the imput jack it was a little wobbly so when i plugged it in and started to headband and flailing my guitar around it would rock the jack causing the volume to swell! Check I prolly bet its the jack!

Strange, that has happened to my MG too (yea... MG...). I was thinking of getting a JCM900 cuz of the same exact fluctuations. Mine would just randomly happen, and cleaning the inputs didn't quite help either.

I couldn't take it to a tech cuz I can't really prove that anything was wrong with it (because it would randomly happen). So yeah, nothing really worked... I guess something's wrong with all Marshalls (and not just MG's?).